Boreholes are sometimes drilled using drilling fluid which has a pressure substantially less than the pressure of the fluid from the formation. This is known as underbalanced drilling. Underbalanced drilling is often used where fluid-bearing formations are known to be delicate and prone to damage, so as to maintain the integrity of the formation. Typically a number of different subterranean structures, with different properties, are drilled through before the actual production formation of interest is reached. The pressure of fluid from the formation will often therefore vary during drilling. It is often important to ensure that the drilling remains underbalanced at all times to minimise formation damage.
Underbalanced drilling is also used generally where, for example, faster drill speeds are required or where the life of a drill bit needs to be extended.
The formations surrounding the borehole can be characterised by a pore pressure, porosity and permeability. When underbalanced drilling, an estimate of the pore pressure is typically made, and the pressure of the drilling fluid is then chosen in an attempt to ensure that underbalanced drilling is achieved at all times. However, the estimates of pore pressure are generally very inaccurate and as such it is often difficult to perform underbalanced drilling with any degree of reliability or control.
The estimate of pore pressure can be used to derive the permeability of the formations, but the estimated pore pressure can be very inaccurate so causing errors in the values of permeability.
The present invention aims to provide a method which supplies more information about formations whilst drilling and aims to enable more controlled underbalanced drilling to be achieved.